A light guiding panel is one of core elements forming a backlight unit. The backlight unit converts light of a tube lamp into light of flat optical sources and is widely used in an apparatus, such as a liquid crystal display (LCD) panel or an advertising display apparatus, which requires a flat light source.
Methods for forming a backlight unit are broken down to a direct-type method and a light guiding panel method according to the location of an optical source. The direct-type method places the optical source directly below a light emitting surface so as to guarantee planar light emission, while the light guiding panel method places an optical source at a side of a light guiding panel so that light which is incident on the side of the panel is guided to be reflected to the front surface of the light guiding panel in order to guarantee planar light emission. The present invention is applied to the light guiding panel method.
A conventional light guiding type backlight unit is comprised of at least a tube type fluorescent lamp, several sheets such as a reflection sheet, a light guiding panel, a diffusion sheet, and a prism sheet, and a mold frame by which the foregoing elements can be assembled into a unit. Among these elements, the light guiding panel is a core element and is made of a transparent acryl resin plate. Light emitted by a tube lamp which is provided along a side of the light guiding lamp, is incident on the side of the panel and then is reflected to the front surface of the light guiding panel by a specific pattern on a bottom surface of the acryl resin plate. The reflection sheet makes the light having passed through the bottom surface of the light guiding panel be reflected into inside of the light guiding panel. The diffusion sheet is put on the front surface of the light guiding panel and makes the light from the front surface be scattered uniformly such that an appropriate amount of light is made to be spread over the front side of the light guiding panel. The prism sheet makes the light from the diffusion sheet be refracted and collected such that the brightness of the surface of the backlight unit can be enhanced.
Among conventional light guiding panel manufacturing technologies, there is a dot pattern printing method using a silkscreen printing technology. In the silkscreen printing method, dots are formed on the bottom surface of a light guiding panel by a silkscreen printing method and minute glass beads are put inside the dots such that light is scattered on the surfaces of glass beads and scattered light passes the top surface of the light guiding panel. This method is a stable technology that has been used for tens of years, but the resolution of a film, and the kind, tension, and angle of gauze during an engraving process are closely related to the dot size and pitch of a light guiding panel desired to be printed. Accordingly, identifying and adjusting these correlations need much know-how. In addition, the printing process is complex and many defections may occur during the printing process. There is another conventional light guiding panel manufacturing technology, or a V-cut method, which uses a machine tool. In the V-cut method, V-shaped grooves are directly made on a transparent acryl resin substrate by a machine tool.
However, in the light guiding panels made by the dot pattern printing method or V-cut method, the scatter light reflected by the dots or V-grooves is emitted to the front surface of the light guiding panel such that the reflection pattern is seen as is on the light guiding panel. That is, since the reflection pattern, which is the reflected dots or V-grooves, is of a size enough to be seen with the naked eye, so-called light spots, in which bright parts or dark parts are locally mixed and coexisting, appear on the front surface of the light guiding panel. In order to remove or minimize the light spots, sheets such as a diffusion sheet, which is made from polymer material being mixed with minute beads, or a prism sheet is generally attached on the front surface of a light guiding panel. If a diffusion sheet is used, an appropriate amount of light is made to be spread such that the uniformity of brightness can improve. However, since the light transmission factor of a diffusion sheet is usually about 68%, employing a diffusion sheet and the like causes an adverse effect of light loss, the optical efficiency is degraded, and the brightness of the backlight is lowered, accordingly. In order to improve optical efficiency, a light guiding panel which does not need a diffusion sheet and the like is required.
For this, a variety of research activities, have been proceeding recently, including researches on a method for directly molding a light guiding panel having a light scattering function by using a mold for manufacturing a transparent resin substrate which is part of a light guiding panel, the mold whose surface is processed by laser, sandblast, or erosion. However, the market related to manufacturing a light guiding panel demands establishment of a mass production system according to automation of light guiding panel production and achievement of production cost reduction no less than achieving the uniformity and stability of brightness distribution of light emitting from a light guiding panel. The conventional technologies for manufacturing light guiding panels cannot satisfy these demands well.